In the prior art, U.S. Pat. No. 3,850,821 teaches a method for manufacturing bearings by mixing acetal polymer or copolymer with 2-15 wt % of a lubricant, melting the mixture at a temperature more than the melting point of the acetal polymer, cooling the mixture to a solid mass, powdering the lubricant containing polyacetal, and molding the same. However, such method has the disadvantage of lowering the manufacturing efficiency of the molded article, since it requires a relatively long time to have the lubricant, such as No. 30 engine oil corresponding to SAE No. 30, be contained into the polyacetal, and also to powder the lubricant containing acetal polymer.
There is another disadvantage which is liable to depress the quality of polyacetal when it is heated for a long time at a temperature of more than the melting point of polyacetal.
Attempts have been made to solve the foregoing disadvantages in the manufacture of the lubricant containing bearing material. Many ways were tried to manufacture the lubricant containing bearing material from the molding material produced by mixing the polyacetal homopolymer or copolymer powder of various grades with lubricant, but most of them were not successful, except when some kinds of oxymethylene copolymer (celcon flakes M 90) were used as the molding material.
However, it has been found at last from experimental results that the value of the bulk factor of the polyacetal as mentioned below, and the molding conditions determine whether or not a satisfactory molded article or bearing is produced, and that such bulk factor value indicates the maximum amount of the lubricant to be contained in the bearing material for satisfactory article production.
This invention relates to bearings or other antifriction elements formed of the lubricant-containing polyacetal and to a method of making such elements. More particularly, this invention relates to providing bearings or other antifriction elements formed of the polyacetal containing 2-12 wt % of the lubricant uniformly dispersed as finely divided particles separated from each other in the resin.
In the specification, the term "polyacetal" means oxymethylene homopolymer or copolymer of trioxane and ethylene oxide.
Polyacetal as well as nylon resin are generally very excellent in low friction; they have a small difference between a kinetic friction and a static friction and also they have good mechanical strength, namely a wear resistance, a creeping resistance, and a fatigue resistance together with desirable chemical properties, namely resistance to oil and other common organic solvents. These advantageous features of the polyacetal in combination with its other merits accounts for the fact that polyacetal is widely used as engineering plastics, particularly in the field of bearings, gears, cams, liners and other mechanical elements which need a substantial antifriction or wear-resistance feature.
It has, however, been found by us that, if the polyacetal is used under such relatively severe conditions as in a vehicle or an industrial machine without any supplement of a lubricant, they will be unable to exhibit satisfactory bearing performance.
To overcome the disadvantages as mentioned above, it is a common knowledge to apply to the sliding surface of the polymer such lubricants as a mineral oil or grease at the beginning of the use but it is impossible to continue the lubricant effectiveness over long time periods. In case that the lubricant exhausts on the sliding surface, the friction heat increases whereby the wear of the bearing material will be accelerated.
It has been found from various experiments that solid lubricants such as graphite, molybdenum disulfide or lead monoxide serve to improve the load carrying capacity but they do not serve to improve a self-lubricating property or the speed characteristic property and also that polyolefins such as polyethylene or polybutene containing such lubricants as well as other plastics compounded with the lubricant containing polyolefin are not suitable for bearings, since the bearings formed of said bearing material generally lack load carrying capacity and are easy to deform and also cannot be used under any severe conditions.
In order to manufacture plastic bearings having good self-lubricating properties, load carrying capacity and speed characteristic property, it is the most essential matter as confirmed from our experiments to provide the bearings formed of polyacetal containing 2-12 wt % of a lubricant which is in the liquid state at the normal room temperature or at the melting point of polyacetal.
However, many difficulties are encountered to cheaply manufacture polyacetal bearings containing the lubricant in an industrial scale as compared with the manufacture of those containing a solid lubricant such as graphite, since the lubricant is separate from the polyacetal since the lubricant may not be uniformly mixed or dispersed through the polyacetal during an injection of an extrusion molding process whereby a satisfactory molding article or bearing is not produced.
It has been proved from experimental results that the separation of the lubricant from the polyacetal during molding tends to be reduced by use of fine polyacetal powders having the bulk factor of 2 to 8, but the mass production of the molded article is difficult resultant from a lack of moldability of the materials.
In order to improve mixing and kneading the polyacetal powders with the lubricant so as to uniformly disperse the lubricant as finely divided particles through the polyacetal molded article to be produced, a screw type injection or extrusion molding machine is used. But satisfactory result(s) is not yet obtained by the reason that the polyacetal powders containing 2 - 12 wt % of the lubricant slips in the feed section of the heating cylinder of the screw type injection molding machine thereby making it difficult to transfer the polyacetal powders to the nozzle section of the heating cylinder. Said disadvantages have been removed by cooling the feed section of the heating cylinder by means of an appropriate cooling apparatus.
The polyacetal powders mixed with the lubricant are fed to the heating cylinder of the molding machine and melted, and the said lubricant must be uniformly dispersed through the polyacetal and maintained in a dispersed state during the injecting or extruding the polyacetal from the nozzle section. Therefore, in case that the lubricant oozes from the polyacetal melt during molding, the oozed lubricant gathers in the space between the polyacetal melt and the inner surface of the heating cylinder, whereby the polyacetal melt slips within the heating cylinder, and is not kneaded by the screw of the molding machine and is not transferred to the nozzle section. It results in the decomposition of the polyacetal melt thereby making it impossible to mold. In case a solid lubricant such as graphite powders and MoS.sub.2 powders which is in the solid state at the melting point of polyacetal are used, the said phenomenon does not occur at all, while in case a lubricating oil which is in a liquid at the normal temperature and paraffin wax which is the liquid state at the melting point of the polyacetal are used, the said phenomenon occurs. When the additional amount of the lubricating oil and the paraffin wax is small, for example less than 2 wt %, it is out of the question that the said lubricant oozes out from the polyacetal. But, when the additional amount of the lubricating oil is 2 - 12 wt % on the basis of the polyacetal, it comes into question. The present invention comprises the two essential conditions or matters, one of which being to use the polyacetal powders having the bulk factor 2 - 8 to obtain the polyacetal molded article having 2 - 12 wt % of the lubricant uniformly dispersed therein, the other of which being to control the temperature of the heating cylinder in a feed section of the screw type or injection type molding machine to that of 40 - 75% of the melting point of the polyacetal in order to uniformly disperse the lubricant through the polyacetal melt during molding and to maintain the lubricant in the dispersed state during molding.
The object of this invention is to provide a lubricant containing polyacetal having superior properties for bearings or other antifriction elements.
Another object of this invention is to provide a lubricant containing polyacetal suitable for bearings or other antifriction elements which polyacetal is highly self-lubricating and has a low coefficient of friction and also is excellent in resistance to wear and in load-carrying capacity and also in speed characteristic property.
Another object of the present invention is to provide an efficient and useful method of molding the lubricant-containing polyacetal having desirable properties for bearings or other antifriction elements.
It has been found that the objects of this invention are accomplished by mixing at a normal room temperature the polyacetal powders having a bulk factor of 2-8 with 2 to 12 wt % of lubricant which is in a liquid state at the normal room temperature or at the melting point of polyacetal and molding the resulting composition by means of a molding machine having controlled temperature zones therein, so that the lubricant may be uniformly dispersed in the polymer as finely divided particles with the assistance of a heat and a pressure during molding.
One of the polyacetal used in this invention is a homopolymer of formaldehyde disclosed in U.S. Pat. No. 2,768,994 specification which does not have a degree of toughness of at least 1, but a reaction rate constant for a thermal degradation at 222.degree. C. of less than 1 wt %/min. and a weight average molecular weight (viscosity method) between 50,000 - 70,000 and a crystalline melting point of 175.degree. C.
Another polyacetal used in this invention is oxymethylene copolymer of trioxane and ethyleneoxide (mole ratio of trioxane/ethylene oxide = 100: about 0.1 - 15) disclosed in U.S. Pat. No. 3,027,352 specification which has a softening point of 162.degree. C., a crystalline melting point of 163.degree. C., a molecular weight average determined by the viscosity method of 50,000 and is called tradenamed "Celcon" sold by Celanese Corporation USA.
The lubricant to be used in this invention is primarily selected from a mineral oil and a synthetic lubricant oil which are in a liquid state at the normal room temperature or may also be selected from paraffins, a higher fatty acid or a higher fatty acid ester, which are in a solid state at the normal room temperature but in a liquid state at the melting point of the polyacetal and grease which is in a jelly state at the normal room temperature. The lubricants may be used solely or mixing one with another. As occasion demands, additives such as lithium stearate, graphite and molybdenum disulfide powders may be added to the polyacetal together with the lubricant. The amount of such additives as graphite and molybdenum disulfide should preferably be limited to less than 5 wt % on the weight basis of the molding composition. Such additive as lithium stearate may be used together with the lubricant in order to improve the dispersibility as well as the retention of the lubricant but the amount of said additive should preferably be limited to less than 3 wt %.
It has been proved that the maximum amount of the lubricant to be contained in the molded article depends on the particle size of the polyacetal powders namely, that the more the particle size of the polyacetal powders decreases, the more the amount of the lubricant to be contained to the molded article will increase.
However, it has been observed that only small amounts of the lubricant may be contained in the polyacetal molded article in spite of using a relatively small apparent particle size of the polyacetal powders, and that a relatively large amount of the lubricant may be contained into the polyacetal molded article, in spite of using a relatively large apparent particle size of the polyacetal powders.
Therefore, it is impossible to estimate the maximum amount of the lubricant to be contained in the polyacetal molded article on the basis of a mesh or a micron of the polyacetal powders owing to the unexpected phenomenon as mentioned above, and also by reason that the mesh or the micron measurement often shows the apparent particle size of the aggregated powders. But as a practical problem, the determination of the actual particle size has many difficulties, especially for small scale manufactures.
Generally speaking, an expression of the particle size of powders is shown as follows:
(a) mesh by the sieve, PA1 (b) micron by the microscopic observation, PA1 (c) surface area of powders (m.sup.2 /gr), PA1 (d) density (or apparent density) (gr/cm.sup.3). PA1 d.sub.2 = 1.42 g/cm.sup.3 polyacetal homopolymer. PA1 D.sub.2 = 1.41 g/cm.sup.3 polyacetal copolymer. PA1 The apparent density is measured by ASTMD : D1895 - 65T, Method A. PA1 10 - 20 parts by weight of glycols for example triethylene glycol were added to 1 - 2 parts by weight of the polyacetal pellets and the resulting mixture was heated to the temperature of 165.degree.- 200.degree. C and then allowed to stand to obtain a fine precipitate. The precipitate was filtered and washed with water or alcohol to remove glycol and then dried in vacuum. Fine beautiful powders of the polyacetal were obtained. The interrelationship between the particle size of said powders and the bulk factor thereof was measured as shown in table 3.
The mesh or the micron measurement is inadequate as the estimation of the particle size of the polyacetal powders as mentioned above and also the surface area is inadequate since the measurement of the particle size is complicated and has need to use a high cost measurement instrument.
The density or the apparent density is seemed to be preferable to indirectly measure the particle size of powders to estimate the amount of lubricant to be contained to the polyamide molded article.
The inventors have researched for a suitable method for forseeing the maximum amount of the lubricant to be contained in the polyacetal molded article on the basis of the particle size of the plastic powders prior to molding where the lubricant is added to plastics not having any affinity for the lubricant for example in a polyacetal. It has been concluded that said method according to the bulk factor is the most practical, simple and reliable estimating method.
The bulk factor is expressed by the following formula 1, wherein D.sub.1 represents the apparent density of the polyacetal powders prior to molding, D.sub.2 represents the inherent density of polyacetal itself as generally reported. EQU Bulk Factor = (D.sub.2 /D.sub.1
It has been discovered from experiments that in order to give the self-lubricating property to the polyacetal molded articles and to improve the load-carrying capacity as well as the speed characteristic property thereof, at least 2 wt % of the lubricant especially more that 3 wt % of the lubricant must be contained in the molded article of the polyacetal so as to uniformly disperse into the polyacetal molded article as finely divided particles. Further, in case of more than 12 wt % of the lubricant being contained in the molded article, it reduces the load-carrying capacity and the moldability of the polyacetal powders and also many voids, or deformation as well as flow marks are easy to create in the molded article thereby depressing the worth of the molded article.
Table 1 shows the relationship between the bulk-factor and the amount of the lubricant to be contained in the polyacetal molded article.
Table 1 ______________________________________ BULK FACTOR Less than 2 2 - 3 3 - 4 4 - 8 ______________________________________ Maximum amount of lubricant less less less less to be than 2 than 5 than 8 than 12 contained (wt%) ______________________________________
If the apparent density of the polyacetal powders is 0.4, the bulk factor of said powders results in 3.5 from formula 1. Then the maximum amount of the lubricant to be contained is estimated to be less than 8 wt % from the table 1.
In this case, if more than 8 wt % of the lubricant for example 10 wt % thereof is added in the polyacetal powders, excess parts of said lubricant will be excluded during molding and make it impossible to mold due to the presence of the excluded lubricant.
The bulk factor influences not only the amount of the lubricant to be contained but also the dispersibility of the lubricant in the polyacetal molded article as well as the moldability of the polyacetal powders, namely the more the bulk factor increases, the more the moldability of the polyacetal powders and the uniformity as well as the dispersibility of the lubricant contained in the polyacetal molded article improves. Therefore, in case that the polyacetal molded article containing 6 wt % of the lubricant is desired to be manufactured, the polyacetal powders having the bulk factor of 3 - 4 may be used, but the polyacetal powders having the bulk factor of more than 4, namely 4 - 8 is preferable to use.
It has been found from our experiments that the polyacetal powder having the bulk factor of 7 - 8 namely the apparent density of 0.18 - 0.2 gr/cm.sup.3 is manufactured according to the following methods as shown in the following item (ii).
In theoretically, the polyacetal powders having the bulk factor of more than 8 may be manufactured, but the maximum value of the bulk factor has been about 8 until now according to our experiments.
The following experiments have been effected by the inventors to obtain polyacetal powders having a bulk factor suitable for containing the lubricant.
(i) Polyacetal pellets (Derlin 500) were pulverized by means of a mechanical pulverizer. The relationship between the particle size (Mesh) of powders manufactured according to the mechanical process mentioned above and the bulk factor was shown in the table 2.
Table 2 ______________________________________ Tyler less than 48 - 65 - 80 - more than Mesh 48 65 80 100 100 ______________________________________ BULK FACTOR (Average Value) 1.60 1.72 1.92 2.08 2.17 ______________________________________
(ii) Polyacetal pellets (Derlin 500) were pulverized according to the following process in the presence of a specific solvent.
Table 3 ______________________________________ Tyler 20 - 48 - 65 - 80 - more than Mesh 48 65 80 100 100 ______________________________________ BULK FACTOR (Average Value) 6.95 7.00 7.00 7.00 7.05 ______________________________________
(iii) Some kind of oxy-methylene copolymer, for example Celcon flakes M 90, has been observed from experimental results to have a relative large bulk factor. The interrelationship between the particle size of the polyacetal powders and the bulk factor is shown in the table 4.
Table 4 ______________________________________ 20 - 48 - 65 - 80 - more than Mesh 48 65 80 100 100 ______________________________________ BULK FACTOR (Average Valve) 4.50 4.55 4.60 4.65 4.76 ______________________________________
In view of said experiments, it has been concluded that the reason why the powders having the same particle size indicate the different bulk factors as shown in the tables 3 and 4 is due to the aggregation of the polyacetal powders. Therefore, even if the amount of the lubricant to be contained into the polyacetal molded article depends on the actual particle size thereof, the estimation of the lubricant to be contained can not be effected on the basis of the particle size of the polyacetal powders measured by the mesh or the micron, since the mesh or the micron does not indicate the actual particle size of the polyacetal powders but the particle size of the aggregated powders thereof.
Table 5 shows the influences of the amount of lubricants upon the bearing performances. In this table, the lubricant used with polyacetal was SAE #30 engine oil. Additives used include lithium stearate and graphite.
Table 5 __________________________________________________________________________ Additives Bearing wt % performance Speci- Poly- Lubri- Bearing Coeffi- Maxi- men mer cant Lithium Graph- temp. cient of mum No. wt % wt % stearate ite .degree. C friction PV value __________________________________________________________________________ 1 Oxy- 85 15 0 0 40 0.04 1,610 2 methy- 87 13 0 0 40 0.04 1,890 lene 3 copoly- 85 12 3 0 40 0.04 2,030 4 mer 88 12 0 0 40 0.04 2,100 5 87 10 2 1 39 0.04 2,100 6 90 10 0 0 40 0.04 2,100 7 90 8 1 1 40 0.04 2,030 8 92 5 0.5 2.5 43 0.05 1,890 9 93 2 0 5 60 0.08 1,400 10 95 2 3 0 45 0.06 1,610 11 98 2 0 0 50 0.08 1,330 12 98.5 1.5 0 0 90 0.13 420 13 99 1.0 0 0 100 0.20 420 14 100 0 0 0 105 0.23 350 15 Oxy- 87 13 0 0 40 0.04 1,820 16 methy- 84 12 3 1 42 0.04 1,960 17 lene 88 12 0 0 40 0.04 2,030 18 homo- 90 8 2 0 40 0.04 2,030 19 poly- 93 4 0 3 46 0.06 1,680 20 mer 93 2 0 5 55 0.09 1,400 21 95 2 3 0 44 0.07 1,610 22 98 2 0 0 53 0.10 1,330 23 99 1 0 0 100 0.20 420 24 100 0 0 0 107 0.24 350 __________________________________________________________________________
The tests were conducted on a thrust type friction-wear testing machine under conditions including a sliding speed of 14 m/min and an accumulated load pressure of 5 kg/cm.sup.2 per 5 minutes.
The bearing temperature and the coefficient of friction were measured after the lapse of 60 - 80 minutes from the beginning of the test in the condition of the accumulated load pressure of 60 - 80 kg/cm.sup.2 and the values thereof were shown by an average value except for specimens Nos. 12, 13, 14, 23 and 24. In said specimens, the values were tested under an integrated load pressure of 15-20 kg/cm.sup.2 after the lapse of 15-20 minutes from the beginning of test and the values thereof were shown by the average one.
The maximum PV value was those calculated by the accumulated load pressure at the moment when a specimen polymer begins to melt.
Next, the method of making the bearings or other anti-friction elements of the polyacetal according to the present invention will be described.
In order to manufacture the molded article of this invention, a commercial screw type injection molding machine or extruding molding machine is used. In the commercial injection molding machine, the single or multiple screw device having various feed throats, for example, vertical, slope or undercut, may be used. The screw construction of full-flighted type having constant pitch and varying channel depth may be used.
The length to the diameter ratio of the screw may be ranged from about 12:1 to about 16:1, generally 14:1 to 16:1.
The screw may be driven at the rate between about 1 and 60 rpm, preferably 30-50 rpm.
If the screw is driven at the rate of more than 60 rpm, it becomes difficult to mold.
The compression ratio may be used at the rate between 2.5:1 and 3.0:1; if said ratio is more than 3.0:1, the desired molding article is not manufactured.
In the commercial extrusion molding machine, the single or multiple screw device having various feed throats, for example, vertical, slope or undercut, may be used.
The screw construction of full flighted type having either constant pitch and varying channel depth or the semi-compression type and varying channel depth may be used.
The semi-compression type means the type having the slope part containing about two pitches between the feed section and the metering sections of the screw.
The length to the diameter ratio may range from 12:1 to about 24:1, generally the range of 18:1 to about 22:1 is employed. The screw may be driven at rates between about 1 and 60 rpm and most preferably between about 30 and 40 rpm. When the screw is driven at the rates of more than 60 rpm, it becomes difficult to mold.
The compression ratio of the range between 2.5:1 and 3.0:1 is generally employed. If said ratio reaches more than 3.1:1, the desired molding article is not obtained.
In order to manufacture the molded article of this invention, it is necessary to arrange a specific cooling apparatus in the feed section F of the heating cylinder 1 of the screw type molding machine having the screw construction as mentioned above and as shown in FIGS. 1 and 2.